The geographical area covered by cellular mobile communication systems is divided into smaller radio areas, i.e. radio cells to attain a better use of the frequency channel by means of the reuse of frequencies. The size of radio cells and the number of channel resources allocated to them (radio channels/traffic channels) vary depending on many factors, such as the required capacity, terrain, the used transmission powers, etc. The object is to estimate these factors as well as possible as early as the planning stage of the cellular network in such a manner that it is possible to provide a service of a sufficient quantity and quality in all parts of the radio network. As network planning is anyway based on estimates and forecasts and the need for capacity is continuously changing by areas, the distribution of loading is continuously monitored in built networks, too and possible problem areas are attempted to be discovered. General information about loading is derived from various call statistics with which even cell-specific accuracy can be attained. As a result, it possible to increase channel capacity in an overloaded cell, or to modify the network in some other way. Various call statistics do not, however, provide any information about how traffic load is distributed geographically inside an individual cell. If this information could be collected from the network, it would enable a more accurate focusing of additional capacity (additional frequencies) to the places of a cell with a great amount of traffic (so-called hot spots) or even to establish a new cell in this place.
In practice, the location of spots hot with traffic requires the location of the individual mobile stations having an ongoing call. Prior art discloses several different methods for locating mobile stations for other purposes than traffic monitoring.
WO 9507587 discloses a method where the mobile station measures the signal strengths of surrounding base stations. The rough location and distance of the mobile stations from each base station is determined on the basis of the signal strengths. A theoretical geographical distance between the transmission and reception is calculated from radio path attenuation which is determined as the difference between the transmitted and received signal strength.
In WO 9205672 the mobile station listens to the control channels of surrounding base stations and selects at least three base stations and establishes a call to them. Each base station calculates its distance to the mobile station by using a timing advance value. The location of the mobile station can be determined by means of at least three calculated distances.
EP-0320913 teaches a location method where base stations whose locations are known transmit identifier signals and an accurate time pulse in sync with one another. The times of arrival of time pulses at different base stations are proportional to the distance between the mobile station and the base station, in which case the location of the mobile station can be determined by using at least three base stations.
These known location methods of mobile stations, intended for locating individual mobile stations, are, however, too complicated and difficult to calculate or even completely unsuitable when the distribution of a great number of calls is to be surveyed by area and time inside a cell. Furthermore, most of them require additional functions from the mobile station and the base stations, cause extra radio traffic, increase signalling load, etc.